Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump for an hydraulic elevator. Each time movement of an elevator car is commanded, then the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction.
One type of elevator starter initially connects the motor windings in a Y-configuration to start the motor and bring it up to speed. Then the windings are reconnected in a delta configuration with full voltage. Other starters change the on time of the solid state switches to ramp up motor current with a fixed connection. Known elevator starters have selector switches for setting a starting current limit setting. Depending on configuration, the setting is adjustable from about 100 percent to 450 percent of the starter's current rating. As a general rule, the higher the setting the lower the start time and conversely, the lower the setting the longer the start time. In an elevator application end users are interested in starting the motor as fast as possible while eliminating power quality issues such as voltage dips and sags and contact or switching transients. Known designs utilize current transformers to read motor current. The current is rectified and filtered before being read by an analog to digital converter in a digital signal processor (DSP). Due to filtering there is a delay between the current on the line and the actual signal the DSP receives.
On some elevator control systems with variable loading the load may be brought up to speed on a particular setting without any voltage dip when the power system is lightly loaded. The same setting may cause the line voltage to dip when the power system is loaded at or near its capacity such as during summer months when power demand is extremely high and brownout conditions are common. In such situations the starting current limit is usually set to the lower setting discussed above. While this ensures that the starter will start the load with minimal voltage fluctuations, additional time is spent starting at the lower current. As can be appreciated, if the starter is not initially set up correctly, then repeated trips back to an installation may be required for further adjustment to eliminate voltage dips.
Starter applications using back up generators can have similar problems. When operating off the line power, the incoming voltages can be reliable and allow for starting currents in excess of 300% of the starters rating. However, when a back up generator is powering the system, then voltage dips can be seen with current limit settings above 200%. In order to allow a system to operate without a excessive voltage dips, the setting for the generator would have to be used even though it would add additional unnecessary time to each start when the system is powered off of line power.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.